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WATCH: Thousand so mourners filed past the coffin of slain Russian opposition leader Boris Nemtsov, before his burial in Moscow on Tuesday. As Stuart Greer reports, there’s deep suspicion that the Kremlin ordered the assassination and the murder will never be solved.

Russian opposition leader Boris Nemtsov’s assassination last Friday came as a shock to many in Russia and around the world, but the slaying of dissenting voices in Russia is hardly unheard of.

Nemtsov was walking with his Ukrainian girlfriend, Anna Duritskaya, on the Great Moskvoretsky Bridge, in sight of the Kremlin, when a gunman shot him four times.

His killer remains at large, and the motive is still a mystery.

WATCH: Unverified footage purporting to show the moment Kremlin critic Boris Nemtsov was killed in Moscow.

WATCH: The woman who was with Nemtsov, when he was gunned down on Friday, has spoken out.

Those familiar with Russian politics say it would serve no purpose for Putin to have ordered Nemtsov’s death, noting his high approval rating, but say the political climate in Russia could have certainly played a role.

“Some people would say it’s hard to imagine this happening in Russia without the involvement of Putin and he could have had some conceivable motives for wanting to get rid of Nemtsov,” said Matthew Light, an assistant professor at the University of Toronto’s Centre for European, Russian and Eurasian Studies.

“[But] the claim that Putin would have done this is just circumstantial. …. No one has any proof that he [Putin] did it and I think the risk [in saying that] is turning Putin into this all-powerful, godfather-like figure,” he said.

“If Putin tolerated him all this time, why would he need to kill him now or somebody need to kill him now?”

Light explained there has been a “deliberate cultivation of extreme nationalism by the Russian government” that has helped far-right groups to thrive. Since the tensions with Ukraine led to Russia’s annexation of Crimea and the backing of separatist factions in the eastern part of that country, Putin has ramped up his rhetoric against his opponents.

“Some people are suggesting it’s possible a killing like this could be carried out by amateurs, so to speak, who are in sympathy with the Kremlin, but not under its orders.”

Boris Nemtsov, a sharp critic of President Vladimir Putin, was killed early Saturday. His death comes just a day before a major opposition rally in Moscow.

Misha Japaridze, File/AP Photo

Canada’s Immigration Minister Chris Alexander, who served as a diplomat in Russia and said he knew Nemtsov well, said there’s little doubt his outspoken views were a factor in the killing.

“Most recently, he had been one of the loudest, and certainly the most prominent, opposition leaders speaking out against Putin’s illegal war in Ukraine,” Alexander told Global News on Monday. “We suspect that he was killed because of those views, as I think almost all Russians assume as well.”

“This is a high-profile, political death of someone who was clearly opposed to Vladimir Putin’s policy in Ukraine [and] to his attempts to thwart free speech in Russia,” he said.

WATCH: Thousands of Russians march to mourn slain opposition leader.

Alexander is among those calling for an independent investigation into Nemtsov’s murder.

Joan DeBardeleben, Chancellor’s Professor in the Department of Political Science and the Institute of European, Russian, and Eurasian Studies at Carleton University, said while the political environment may have been a factor that led to Nemtsov’s death, she’s “doubtful” whether all the details will ever be known.

“While President Putin’s statement that he’s going to, himself, personally oversee that people are brought to justice is intended to show concern, it certainly is not the equivalent of an independent investigatory process,” DeBardeleben said.

“Given the past history of resolving these types of cases, I wouldn’t be too optimistic there would be a very clear attribution of responsibility,” she said.

There’s a history of those who criticized the Russian government and expose corruption or human rights violations in Russia winding up dead.

Since Putin first came to power in 2000, prominent human rights activists, journalists and opponents have been murdered or died under suspicious circumstances, both in Russia and abroad.

Paul Klebnikov, 41

Paul Klebnikov, the editor of Forbes Magazine’s Russian edition and author of a book about tycoon Boris Berezovsky, was shot to death in July 2004 in Moscow.

/Misha Japaridze, File/AP Photo

U.S. journalist and editor of Forbes RussiaPaul Klebnikov was shot to death in 2004, in what the Committee to Protect Journalists (CPJ) labeled a “contract-style murder.” He investigated corruption and organized crime in Russia, and reported on the Chechen conflict.

Boris Berezovsky, 67

In this Wednesday, March, 10, 2010 file photo, self-exiled oligarch Boris Berezovsky leaves the High Court in London after winning his libel case against a Russian broadcaster that accused him of masterminding the murder of a former Russian agent in London.

Alastair Grant, File/AP Photo

Berezovsky, an oligarch and “Kremlin insider,” was a friend and mentor of Putin’s in the 1990s. But he quickly saw the relationship disintegrate once Putin became president. He became a vocal critic and went into exile in England, where he was found dead in 2013. A coroner’s inquest last year found no evidence he had been murdered.

Alexander Litvinenko, 44

In this Friday, May 10, 2002 file photo Alexander Litvinenko, former KGB spy and author of the book ”Blowing Up Russia: Terror From Within”, is photographed at his home in London.

Anna Politkovskaya, 48

Russian human rights advocate, journalist and author Anna Politkovskaya at a book fair in Leipzig, eastern Germany, where she presented her book titled ‘In Putin’s Russia’.

Jens Schlueter (AFP)/Getty Images

Anna Politkovskaya was another critic of Putin whose life was cut short. She was shot on her way into her home in 2006. She had also reported extensively on human rights violations in Chechnya. Last year, five Chechen men were convicted of murdering the Novaya Gazeta reporter.